How did the treatise Musica enchiriadis describe early written counterpoint?

The late 9th-century treatise Musica enchiriadis provides some of the earliest known documentation of written counterpoint, a practice referred to as organum.12

In this manual for singers, the text describes the combination of a plainchant melody, known as the vox principalis (principal voice), with a second part called the vox organalis (organal voice). The treatise primarily details the performance of this second voice in parallel motion at the interval of a perfect fourth or fifth below the principal melody.123

While early examples often focused on this parallel movement, the Musica enchiriadis also provides evidence of a more evolved practice by describing and illustrating two-part singing that incorporates similar and contrary motion. This early form of polyphony was characterized by a note-against-note style, which served as the foundation for what would later be termed counterpoint.12